U.S. Geological Survey

Water Resources Investigation Report 03-4142

by Kari A. Winfield

Abstract

The subsurface at the Idaho National Engineering and Environmental Laboratory
(INEEL) is complex, comprised primarily of thick, fractured basalt flows interbedded
with thinner sedimentary intervals. The unsaturated zone can be as thick as
200 m in the southwestern part of the INEEL. The Vadose Zone Research Park (VZRP),
located approximately 10 km southwest of the Idaho Nuclear Technology and Engineering
Center (INTEC), was established in 2001 to study the subsurface of a relatively
undisturbed part of the INEEL. Waste percolation ponds for the INTEC were relocated
to the VZRP due to concerns that perched water within the vadose zone under
the original infiltration ponds (located immediately south of the INTEC) could
contribute to migration of contaminants to the Snake River Plain aquifer.

Knowledge of the spatial distribution of texture and hydraulic properties is
important for developing a better understanding of subsurface flow processes
within the interbeds, for example, by identifying low permeability layers that
could lead to the formation of perched ground-water zones. Because particle-size
distributions are easier to measure than hydraulic properties, particle size
serves as an analog for determining how the unsaturated hydraulic properties
vary both vertically within particular interbeds and laterally within the VZRP.
As part of the characterization program for the subsurface at the VZRP, unsaturated
and saturated hydraulic properties were measured on 10 core samples from six
boreholes. Bulk properties, including particle size, bulk density, particle
density, and specific surface area, were determined on material from the same
depth intervals as the core samples, with an additional 66 particle- size distributions
measured on bulk samples from the same boreholes.

From lithologic logs of the 32 boreholes at the VZRP, three relatively thick
interbeds (in places up to 10 m thick) were identified at depths of 35, 45,
and 55 m below land surface. The 35-m interbed extends laterally over a distance
of at least 900 m from the Big Lost River to the new percolation pond area of
the VZRP. Most wells within the VZRP were drilled to depths less than 50 m,
making it difficult to infer the lateral extent of the 45-m and 55-m interbeds.
The 35-m interbed is uniform in texture both vertically and laterally; the 45-m
interbed coarsens upward; and the 55-m interbed contains alternating coarse
and fine layers. Seventy-one out of 90 samples were silt loams and 9 out of
90 samples were classified as either sandy loams, loamy sands, or sands. The
coarsest samples were located within the 45-m and 55-m interbeds of borehole
ICPP-SCI-V-215, located near the southeast corner of the new percolation pond
area.

At the tops of some interbeds, baked-zone intervals were identified by their
oxidized color (yellowish red to red) compared to the color of the underlying
non-baked material (pale yellow to brown). The average geometric mean particle
diameter of baked-zone intervals was only slightly coarser, in some cases, than
the underlying non-baked sediment. This is likely due to both depositional differences
between the top and bottom of the interbeds and the presence of small basalt
clasts in the sediment. Core sample hydraulic properties from baked zones within
the different interbeds did not show effects from alteration caused during basalt
deposition, but differed mainly by texture.

Saturated hydraulic conductivities (Ksat) for the 10 core samples ranged from
10-7 to 10-4 cm/s. Low permeability layers, with Ksat values less than 10-7
cm/s, within the 35-m and 45-m interbeds may cause perched ground-water zones
to form beneath the new percolation pond area, leading to the possible lateral
movement of water away from the VZRP.

CONTENTS

Abstract

Introduction

Previous investigations

Purpose and scope

Acknowledgments

Geohydrologic setting

Field and laboratory methods

Drilling and core collection

Sample preparation

Hydraulic properties

Saturated hydraulic conductivity

Unsaturated hydraulic properties

Bulk properties

Bulk density, particle density, and porosity

Particle-size distribution

Specific surface area

Results and discussion

Laboratory measurements

35-m, 45-m, and 55-m interbeds

Spatial variation of interbed properties

Baked-zone intervals

Summary

References cited

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For additional information write to:

U.S. Geological Survey

INEEL, MS 1160

P.O. Box 2230

Idaho Falls, ID 83403

A printed copy of this publication may be purchased from the
Branch of Information Services Section, Denver Federal Center, Box 25286, Denver,
CO 80225-0286, or call 1888-ASK-USGS for ordering information.